1. Field of the Invention
The invention relates to detecting an output voltage of an LED driving device and more particularly to detecting an output voltage of a driving device that uses a constant current source to drive an LED.
2. Description of the Related Art
Ways to drive a light-emitting diode (LED) can be classified into three types: a constant voltage source type, a constant current source type and a pulse type. Advantages of the constant voltage source type driver are low costs and uncomplicated external circuitry; but, a disadvantage is luminance inconsistency of LEDs. Generally, the constant current source type driver can overcome the problem of luminance inconsistency of LEDs. For driving of the constant current source type driver, LEDs are connected in serial so as to assure the LEDs of luminance consistency. In addition, the constant current source type driver can avoid the condition where the reliability of the driving is influenced by the driving current when the driving current exceeds a largest rated value.
When an LED fails, the output voltage of the LED driving device is different from a normal operating voltage. Therefore, a voltage detecting device for detecting whether the output voltage of the LED driving device is normal or not is needed. Particularly, when the power of a system has to be lowered due to heat dissipating concerns, the voltage of power source is lowered. In the case, if the output voltage of the LED driving device is lower than a normal operating range, the output current may be abnormal, and the abnormal output current has to be detected.
FIG. 1 is a schematic diagram of an LED device showing a known way to detect an output voltage of a driving circuitry. The LEDs LED1-N are connected in serial and are coupled to an LED driving circuitry. The LED driving circuitry comprises a driving device 108 and a comparator CP1. In this way, an output terminal a of the driving device 108 of the LED driving circuitry is directly coupled to a negative terminal of the comparator CP1, so that the voltage of the output terminal a can be compared with a reference voltage VREF coupled to a positive terminal of the comparator CP1. When the voltage of the output terminal a is lower than the reference voltage VREF, a detecting signal Flag1 output by the comparator CP1 is at a high voltage level. The condition where the detecting signal Flag1 is at a high voltage level means that the voltage of the output terminal a of the driving device is too low and that the output current lout may be abnormal.
In order to connect a plurality of LEDs in serial, a voltage VLED is usually much higher than voltage VDD. Therefore, when a pulse-width modulator PWM controls a switch Si to be off, the comparator CP1 has to be able to receive a high-voltage-level voltage. However, the area of the circuitry of the comparator CP1 is larger when using high voltage elements to realize the comparator CP1. Furthermore, an input offset voltage is higher. Therefore, the detecting way as shown in FIG. 1 is not accurate when detecting the voltage of the output terminal a of the driving device.
FIG. 2 is a schematic diagram of an LED device showing another known way to detect an output voltage of a driving circuitry. In this case, in order to use low voltage elements to realize the comparator CP2, the resistances R1 and R2 are used to divide voltage so as to divide the voltage of an output terminal b of a driving device 208 by (VLED/VSS). Accordingly, when the pulse-width modulator PWM controls a switch S2 to be off, voltage of a terminal c connected to a negative terminal of the comparator CP2 is ensured to be lower than or equal to the voltage VDD.
In the way shown in FIG. 2, though low voltage elements can be used to realize the comparator CP2 and thus the input offset voltage is smaller, the influence caused by the input offset is enlarged by (VLED/VSS) times. The greater the amount of LEDs, the higher the voltage VLED is, and thus the bigger the influence caused by the input offset is. In addition, there is also an error caused by mismatch of the resistances R1 and R2 in the way shown in FIG. 2.